Pulp washing is a key operation in a pulping line. There are many different types of washing apparatuses available, some of which are based on press washing and comprise means for pressing the pulp to remove liquid. After pressing, the pulp can, if suitable, be diluted to a desired consistency.
A well-known washing apparatus is a twin-roll press of the general type disclosed in U.S. Pat. No. 3,980,518, for example. It has two counter-rotating rolls with perforated outer surfaces. A web of pulp is formed on the respective rolls and is transported in the direction of rotation in a vat partially surrounding the rolls, to the so-called press nip between the rolls. The liquid removed from the pulp, i.e. the filtrate, passes through the perforated roll surface in a radial inwards direction and is led to the ends of the press roll, where it is output. Washing liquid or other treatment liquid may be supplied to the pulp web through inlets in the vat. The twin-roll press uses the washing principles of displacement, where dirty liquid (liquor) in the pulp is replaced by cleaner wash liquid added to the vat, and pressing, where dirty liquid is pressed (squeezed) out from the pulp, in particular at the press nip.
The incoming pulp can be distributed lengthwise onto the respective press rolls by means of a distribution device, for example by using a rotating screw, such as the device shown in European Patent No. 1,229,164 B1, or the device shown in Swedish Patent No. 532,366 C2. There is a problem with the distribution of pulp along the total length of the press roll, with a danger that the end parts of the press rolls operate without pulp.
In Swedish Patent No. 516,335 a device is described for feeding cellulose pulp, in the form of a pulp web. In this device the outlet includes restrictions in the form of holes, which are arranged along the generator of the envelope surface of the inlet box. The holes are preferably arranged so that their diameter is smaller than the distance between them. In that way, the pressure is maintained in the inlet box such that the pulp is forced out of the outlet and is uniformly distributed along the width of the pulp web. The holes have, however, a tendency to plug.